Thermoplastic vulcanizates (“TPVs”) comprise blends of dynamically cured rubbers and thermoplastic resins where the rubber is dispersed within the thermoplastic resin phase as finely-divided rubber particles. TPVs have advantageously demonstrated many of the properties of thermoset elastomers, yet are processable as thermoplastics. As such, elastomeric articles are often extruded from TPVs using thermoplastic extruding techniques.
TPVs have been prepared by dynamically vulcanizing the rubber in the presence of a curative, such as a phenolic resin, while the rubber is being mixed with a thermoplastic resin. Often cure accelerators or activators are used to aid in the vulcanization process. For example, U.S. Pat. No. 4,311,628 illustrates TPVs that are produced using phenolic resin cure systems that include a phenolic curing resin and a cure accelerator, such as stannous chloride.
Stannous chloride is a solid material and is supplied in the form of a powder. The powder can be difficult to handle and is not easily metered into the reactors where dynamic vulcanization takes place. For example, powdered stannous chloride is hygroscopic and has a tendency to compact and clump if exposed to moisture in the air. Further compaction can occur within the feeder which can result in inconsistent addition of the stannous chloride to the extruder and fouling or plugging of the feeder. In such cases, removal of blockages within the feeder may result in stoppage of the TPV production line or may require a new feeder. Additionally, the powdered stannous chloride can be difficult to disperse throughout the blend of the rubber and thermoplastic resin. Uneven dispersion of the stannous chloride into the TPV production process can affect the cross-linking of the rubber phase during vulcanization.
There is a need for methods of making TPVs that minimize or eliminate issues arising from the addition of powdered cure accelerators, and a method which increases the consistency of the addition of cure accelerators to the vulcanization process. There is also a need to provide methods of making TPVs that have more consistent cure.
GB Patent Nos. 2455981 B1 and 2489123 B1, GB Patent Application Publication No. 2510930, and GB Patent Application No. 1312608, describe masterbatches of stannous chloride and polypropylene.
PCT Publication No. WO 2011/081746 has attempted to overcome some of these difficulties by supplying the stannous chloride to the vulcanization process as a masterbatch with polypropylene. However, there can still be uneven dispersion of the stannous chloride within the rubber phase.